1. Field of the Invention
The present invention relates to a method for forming a Cu diffusion barrier metal for metal wiring structures that can be favorably used in the creation of fine semiconductor elements in general, as well as a structure of such Cu diffusion barrier metal.
2. Description of the Related Art
Ru film is drawing the attention for its property to improve the adhesion with Cu, when a Ru film is formed at the interface between Cu and barrier metal in a Cu wiring structure which is the main wiring structure used in high-speed logic elements such as MPUs, and thereby significantly enhance the reliability of wiring. Methods are being studied to form a Ru film that provides a Cu wiring liner on a TaN film or WN film and then form a Cu film on top of the Ru film (one example of such method relating to a combination of Ru and TaN is described in C. C. Yong et al., “Physical, Electrical, and Reliability Characterization of Ru for Cu Interconnects,” IITC 2006, pp. 187-189).
A Ru/TaN laminated film, whose utilization as a Cu wiring liner is being examined, has a smaller Cu wiring volume as the film becomes thicker, and as a result the Cu wiring resistance tends to become higher. Accordingly, any attempt to reduce the high resistivity resulting from the reduced Cu wire size due to a smaller wiring width requires the Cu wiring liner film to be made thinner. As a result, the industry is paying attention to the atomic layer deposition method, which can be used to form a film offering greater coverage than when the conventional PVD method is used.
When it comes to adhesion, traditionally the PVD method causes physically accelerated ions to strike the surface and therefore a PVD-Ta film or PVD-TaN film constituting a Cu liner can be formed in a manner similar to driving in a wedge. This means that even when a PVD-TaN film is formed on a Cu film of bottom-layer wiring, the TaN film is formed in a manner biting into the Cu film and consequently good adhesion can be achieved. On the other hand, it has been confirmed that a TaN film formed by the atomic layer deposition method would result in lower adhesion with Cu wiring. When a film is formed by means of chemical reaction, unlike when PVD is used an area where different atoms are mixed is not formed between the Cu wiring in the bottom layer and the metal film constituting a Cu liner. This makes it more difficult, than under the PVD method, to ensure good adhesion when a TaN film or TaNC film is formed as a Cu barrier metal. It is expected that good adhesion can be achieved by inserting a Ru film between a Cu film and TaN film or TaNC film.
When forming a multilayer Cu wiring structure, many via holes need to be formed as connection holes with which to connect the top and bottom Cu wirings. Since a dual damascene structure is used in general, the following explanation assumes use of a dual damascene structure. A Cu barrier film is formed on Cu wiring via holes and trenches in the bottom layer, and then a Cu wiring is formed. This is to prevent diffusion of Cu into the inter-layer insulation film and consequent increase in leak current and failed insulation. If the adhesion of Cu with this Cu barrier metal is poor, however, the Cu film will separate in the reliability test and voids will be formed. Accordingly, it is desirable to form a Ru film at the interface between Cu wiring and Cu barrier metal as shown in published examples. However, traditionally forming a Ru film was difficult over the exposed areas of Cu wiring at the bottom of via holes because normally the adhesion is poor at the interface between inter-layer insulation film and Ru. As a way to solve this problem, Se-Hum Kwon presented a paper entitled, “Plasma-Enhanced Atomic Layer Deposition of RuTiN Thin Films for the Application of Copper Diffusion Barrier” at ALD Conference 2004. In this paper, Kwon showed that by adding Ru to a TiN film traditionally used as a copper diffusion barrier film, adhesion with copper could be improved. Hynix Semiconductor Inc. also describes in U.S. Pat. No. 6,800,567 a method for forming a RuTiN film or RuTaN film as a barrier metal film by means of the atomic layer deposition method, where it is self-evident that RuTaN can also be used as a Cu diffusion barrier in a similar manner. In addition, Seong-Jun Jeong et al. presented a paper entitled, “Plasma-Enhanced Atomic Layer Deposition of Ru—TaN Thin Films for the Application of Cu Diffusion Barrier” at ALD Conference 2006. In this paper, Jeong et al. proposed forming a metal alloy film constituted by Ta and Ru between the bottom-layer Cu film and top-layer Cu film by repeating a step to introduce Ta material, step to purge Ta material, step to introduce hydrogen gas and apply high-frequency plasma, step to purge hydrogen gas, step to introduce Ru material (Ru(EtCp)2), step to purge Ru material, step to introduce hydrogen and nitrogen gases and apply high-frequency plasma, and step to purge ammonia gas. In this case, the resulting formation of an alloy of Ta and Ru is shown to achieve good adhesion with the Cu films at the top and bottom and also with dielectric layers.
In addition, Korean Patent Application No. 10-2005-0103373 describes application, as a Cu barrier film, of a film containing amorphous Ru and Ta, wherein such film is formed by repeating the first atomic layer deposition process comprising a step to introduce Ru material, step to purge Ru material, step to introduce ammonia gas and apply high-frequency plasma and step to purge ammonia gas, as well as the second atomic layer deposition process comprising a step to introduce Ta material, step to purge Ta material, step to introduce hydrogen gas and apply high-frequency plasma and step to purge hydrogen gas.
On the other hand, U.S. Pat. No. 6,703,708 proposes a method for changing the Cu, W and N composition of a Cu barrier film in the depth direction, or specifically a method for changing the composition in the depth direction using the atomic layer deposition method in such a way that the Cu content increases at the surface and the W and N contents increase in the bottom layer.